This section is intended to introduce the reader to various aspects of art, which may be associated with exemplary embodiments of the present techniques, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with information to facilitate a better understanding of particular aspects of the present techniques. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Tubular conduits, often referred to as casing or liners, are inserted into boreholes following the drilling of the borehole. In some cases, insertion of these tubular conduits is problematic due to the characteristics of the borehole. Characteristics of the borehole that can make insertion difficult or impossible include high friction between the borehole wall and tubular conduit, high inclination of the borehole, extended horizontal reach of the borehole relative to the mudline or surface location of the well, great depth of the borehole relative to the structural capacity of the surface equipment used to install the conduit, and a subsurface trajectory that features frequent or relatively severe changes in well angle or direction.
One method currently used to install tubulars in boreholes that feature these characteristics is to fill a section of the tubular with a fluid (a liquid or a gas) that has a lower density than the liquid contained inside the borehole. As the tubular is lowered into the borehole, this difference in fluid density provides partial or complete buoyancy of the tubular section containing the lighter fluid. This buoyancy reduces the forces resisting or preventing conduit insertion and thus aids in and allows conduit insertion. More specifically, a plug is placed at the distal end of the tubular, and the tubular is inserted into the wellbore while filling the tubular section with a light fluid (relative to the liquid in the borehole).
After insertion of a significant amount of fluid-filled tubular filled with light fluid or gas into the wellbore, a second or proximal plug is placed within the tubular to trap the light fluid in place. The actual amount can be up to a few kilometers (a few thousand feet) depending upon the specific geometry of the borehole. This section of tubular is buoyed by the heavier fluid in the borehole as it is inserted into the borehole using tubulars. The tubulars can be further inserted into the well borehole with either additional casing or pipe used as an insertion string which are attached to this section of tubular above the proximal plug and contain fluid typically more dense than the light fluid of the buoyed section. An example illustration of this method is described in detail in U.S. Pat. No. 5,117,915.
Another method currently used to install tubulars in boreholes that feature these characteristics is to fill an annulus between a concentric insertion tubular string and the casing or liner with a fluid. The fluid has a lower density than the liquid contained inside the borehole. Similar to the method described above, the difference in fluid density in this insertion-string-by-casing annulus and the density of the fluid in the borehole provides partial or complete buoyancy of the tubular section as it is inserted into the borehole. An example illustration of this method is also described in detail in U.S. Pat. No. 5,117,915.
While these existing methods can be effective in installing tubulars in boreholes that feature these characteristics there are some difficulties associated with these existing methodologies. Specifically, the light fluid provides buoyancy to the tubular at a pressure that is less than that in the wellbore. This can lead to structural collapse of the tubular and loss of well utility.
For instance, if the fluid is a gas, then by conventional flotation methods the pressure in the buoyed interval is essentially atmospheric. Further, gases at near-atmospheric pressure are very compressible. As such, the inserted tubular's resistance to collapse should be provided by the tubular alone. There is no internal pressure to help counteract the external pressure that works to crush the tubular. If the fluid is a compressible liquid (such as, oil or diesel), the pressure in the buoyed portion of the tubular may be above atmospheric pressure but still below the in-wellbore pressure. As such, the inserted tubular's net collapse resistance is less than it may be if open to surface and filled with the same mud as is in the wellbore annulus. The net collapse resistance includes both the mechanical strength of the tubular wall and the internal pressure in the tubular.
Also, the wall thickness of the inserted tubular has an effect on the difficulty associated with floating a casing or liner into a deviated wellbore interval. Specifically, the thicker the wall in the floated interval, the heavier the pipe in the floated interval. Increasing the wall thickness increases the weight which leads to increased drag for a fixed fluid density in the annulus. Increased drag can prevent insertion of a floated casing or liner into a deep or deviated wellbore interval. Therefore, it is advantageous from an insertion standpoint to use casing or liner with thinner wall. However, reducing a thickness exacerbates the tubular collapse problem associated with the conventional method. The thinner the wall, the less capacity the tubular has to resist collapse.
Accordingly, there is a need for an improved tubular insertion methodology that preferably allows buoyant insertion of tubulars without concern for collapse due to pressure differences in and out of the tubular.